The manufacture of semiconductor devices often requires the formation of electrical conductors on semiconductor wafers. For example, electrically conductive leads on the wafer are often formed by electroplating (depositing) an electrically conductive layer such as copper on the wafer and into patterned trenches.
Electroplating involves making electrical contact with the wafer surface upon which the electrically conductive layer is to be deposited (hereinafter the "wafer plating surface"). Current is then passed through a plating solution (i.e. a solution containing ions of the element being deposited, for example a solution containing Cu.sup.++) between an anode and the wafer plating surface (the wafer plating surface being the cathode). This causes an electrochemical reaction on the wafer plating surface which results in the deposition of the electrically conductive layer.
To minimize variations in characteristics of the devices formed on the wafer, it is important that the electrically conductive layer be deposited uniformly (have a uniform thickness) over the wafer plating surface. However, conventional electroplating processes produce nonuniformity in the deposited electrically conductive layer due to the "edge effect" described in Schuster et al., U.S. Pat. No. 5,000,827, herein incorporated by reference in its entirety. The edge effect is the tendency of the deposited electrically conductive layer to be thicker near the wafer edge than at the wafer center.
To offset the edge effect, Schuster et al. teaches non-laminar flow of the plating solution in the region near the edge of the wafer, i.e. teaches adjusting the flow characteristics of the plating solution to reduce the thickness of the deposited electrically conductive layer near the wafer edge. However, the range over which the flow characteristics can be adjusted is limited and difficult to control. Thus, it is desirable to have a method of offsetting the edge effect which does not rely on adjustment of the flow characteristics of the plating solution.
Another conventional method of offsetting the edge effect is to make use of "thieves" adjacent the wafer. By passing electrical current between the thieves and the anode during the electroplating process, electrically conductive material is deposited on the thieves which otherwise would have been deposited on the wafer plating surface near the wafer edge where the thieves are located. This improves the uniformity of the deposited electrically conductive layer on the wafer plating surface. However, since electrically conductive material is deposited on the thieves, the thieves must be removed periodically and cleaned adding to the maintenance cost and downtime of the apparatus. Further, additional power supplies must be provided to power the thieves adding to the capital cost of the apparatus. Accordingly, it is desirable to avoid the use of thieves.
Nonuniformity of the deposited electrically conductive layer can also result from entrapment of air bubbles on the wafer plating surface. The air bubbles disrupt the flow of ions and electrical current to the wafer plating surface creating nonuniformity in the deposited electrically conductive layer. One conventional method of reducing air bubble entrapment is to immerse the wafer vertically into the plating solution. However, mounting the wafer vertically adds complexity and hinders automation of the electroplating process. Accordingly, it is desirable to have an apparatus for electroplating a wafer which allows the wafer to be immersed horizontally into the plating solution and yet avoids air bubble entrapment.